1. Field of the Invention
The present invention relates to a load control apparatus and system for performing data communication adopting power line communication (PLC) method.
2. Description of Related Art
Japanese Patent Publication laid-open No. 2004-120740 discloses a load control system mounted on a vehicle to perform data communication with the use of amplitude shift keying (ASK) method and PLC method. In the ASK method, either value “0” or value “1” is expressed dependently of largeness/smallness in amplitude of a communication signal. While, according to the PLC method, communication is effected through power lines for electric power. In detail, communication signals and drive powers are superimposed on the power lines. FIG. 1 shows a load control system 100 in the conventional art.
The load control system 100 comprises a master station 200, a slave station 300, a power line (i.e. +B line) 400 and a ground line 500. The power line 400 is supplied with direct-current electricity affording a driving electric power. The master station 200 includes a control part 201, a PLC part 202, a branch line 203, an impedance element 204 and a circuit power part 205. While, the slave station 300 includes a PLC part 301, a branch line 302, an impedance element 303, a circuit power part 304, relays 305a˜305d, a motor 306, a control part 307 and a condenser 308. In the load control system 100 constructed above, the master station 200 and the slave station 300 are communicated with each other by means of communication signals, allowing a driving of the motor 306.
By combining signals representing value “0” (each referred to as “0-signal” after) with signals representing value “1” (each referred to as “1-signal” after), the control part 201 produces a master signal relating to an operation content of the motor 306 and further outputs the master signal to the PLC part 202.
The PLC part 202 produces a high frequency signal having predetermined frequency and amplitude (i.e. carrier signal). For the 0-signal forming the master signal, the PLC part 202 makes amplitude of the carrier signal less than a predetermined reference value. While, for the signal “1” forming the master signal, the PLC part 202 makes amplitude of the carrier signal more than the predetermined reference value (i.e. ASK modulation of the master signal). In this way, the PLC part 202 converts the master signal to a communication signal and further outputs it to the power line 400. Additionally, the PLC part 202 also receives a communication signal supplied from the slave station 300 through the power line 400. On receipt of the communication signal, the PLC part 202 converts a signal having its magnitude larger than a reference value to the signal “1” and also converts a signal having its magnitude smaller than the reference value to the signal “0” (i.e. ASK modulation of the communication signal). In other words, the PLC part 200 converts the communication signals to slave signal and outputs it to the control part 201. Note that the slave signal is one produced by the control part 307, representing a completion of the operation of the motor 306 or the like.
The branch line 203 is connected to the power line 400, while the circuit power part 205 is connected to the branch line 203 through the impedance element 204. The circuit power part 205 produces desired electric power from direct-current electricity supplied through the branch line 203 and supplies the electric power to the control part 201 and the PLC part 202. The control part 201 and the PLC part 202 are driven by the electric power supplied from the circuit power part 205.
The PLC part 301 receives a communication signal through the power line 400, converts the communication signal to a master signal by ASK demodulation and outputs the master signal to the control part 307. While, the PLC part 301 also converts a slave signal supplied from the control part 307 to a communication signal by ASK demodulation and outputs the communication signal to the power line 400. In FIG. 2, (a) shows the master signal (i.e. data Tx) produced by the control part 201, (b) shows the communication signal corresponding to the master signal and (c) shows the master signal (i.e. data Rx) produced by the PLC part 301 demodulating the communication signal.
In the slave station 300, the branch line 302 is connected to the power line 400, while the circuit power part 304 is connected to the branch line 302 through the impedance element 303. The circuit power part 304 produces desired electric power from direct-current electricity supplied through the branch line 302 and supplies the electric power to the control part 307 and the PLC part 301. The control part 307 and the PLC part 301 are driven by the electric power supplied from the circuit power part 304.
The relays 305a, 305b have their one ends connected to the branch line 302 through the impedance element 303. The other end of the relay 305a is connected to one end of the relay 305c and one end (terminal) of the motor 306. The other end of the relay 305b is connected to one end of the relay 305d and the other end (terminal) of the motor 306. The other ends of the relays 305c, 305d are connected to the ground line 500.
The motor 306 is driven in rotation by direct-current electricity flowing in the power line 400. In detail, when the relays 305a, 305d are activated (ON), the direct-current electricity flows from the one end (terminal) of the motor 306 to the other end (terminal). When the relays 305b, 305c are activated (ON), the direct-current electricity flows from the other end (terminal) of the motor 306 to the other end (terminal). Therefore, a rotating direction of the motor 306 when the relays 305a, 305d are activated (ON) is opposite to a rotating direction of the motor 306 when the relays 305b, 305c are activated (ON). For example, such rotations of the motor 306 are utilized to drive a powered window (not shown).
The control part 307 controls the operation of the motor 306, based on the master signal supplied from the PLC part 301. In detail, the control part 307 activates either one pair of the relays 305a, 305d or another pair of the relays 305b, 305c to rotate the motor 306 and inactivates all of the relays 305a, 305b, 305c, 305d to stop a rotation of the motor 306. The control part 307 produces the slave signal representing a completion of the operation of the motor 306 or the like and outputs the slave signal to the PLC part 301.
The condenser 308 has one end connected to the branch line 302 through the impedance element 303 and the other end connected to the ground line 500. The condenser 308 introduces noise contained in the direct-current electricity into the ground line 500 to remove the noise from the direct-current electricity.
We now describe the reason why the circuit power parts 205, 304 and the motor 306 are connected to the branch lines 203, 302 through the impedance elements 204, 303, respectively. In the load control system 100, it is necessary to supply the circuit power parts 205, 304 and the motor 306 with the direct-current electricity flowing in the power line 400. In the load control system 100, therefore, the branch lines 203, 302 are connected to the power line 400, while the circuit power parts 205, 304 and the motor 306 are connected to the branch lines 203, 302 respectively, whereby the direct-current electricity flowing in the power line 400 can be supplied to the circuit power parts 205, 304 and the motor 306.
However, it is noted that the above structure causes the communication signal to flow in the branch lines 203, 302 also. Thus, unless impedances from the branch lines 203, 302 up to the circuit power parts 205, 304 and the motor 306 are ensured, the communication signal in case of a current signal would be easy to flow in the branch lines 203, 302, so that amplitude of the communication signal flowing in the branch lines 203, 302 get larger. For a high-frequency component such as the communication signal, the condenser 308 connected to the power line 400 can be regarded as a conducting wire (i.e. one kind of short circuit). Although it is not shown in the figure, a condenser similar to the condenser 308 is connected to the branch line 203. Accordingly, in case of the communication signal of a voltage signal, a potential of the power line 400 (in detail, potential related to a high-frequency component and corresponding to amplitude of the communication signal flowing in the power line 400) would fall unless impedances from the branch lines 203, 302 up to the circuit power parts 205, 304 and the motor 306 are ensured. In detail, the potential of the power line 400 would fall close to a ground potential. Thus, if impedances from the branch lines 203, 302 up to the circuit power parts 205, 304 and the motor 306 are not ensured, there is a possibility that the amplitude of the communication signal for the PLC parts 202, 301 get smaller so as to damage accuracy of communication.
From the reason mentioned above, the circuit power part 205 is connected to the branch line 203 through the impedance element 204, while the circuit power part 205 and the motor 306 are connected to the branch line 302 through the impedance element 303. Consequently, the impedances from the branch lines 203, 302 up to the circuit power parts 205, 304 are ensured.
However, as the impedance element 303 is supplied with not only direct-current electricity for the circuit power part 304 but also direct-current electricity for the motor 306, the same element 303 is required to cope with such great current. From this point of view, the impedance element 303 is large-sized and therefore, the load control system 100 is also large-sized, causing both weight and manufacturing cost of the system 100 to be increased.